A general shape of a fuel injection nozzle is shown in FIG. 26. The fuel injection nozzle is formed with a hollow hole with an inside diameter of 2 to 4 mm in the axial direction. A fuel jet is formed at the tip end of the hollow hole, and an undercut serving as a fuel reservoir is formed in a far portion of the hollow hole.
It is when the diameter of the inner peripheral portion of the member is 10 mm at the smallest that the undercut can be formed by machining in the inside portion of the member. In order to form the undercut in the inner peripheral portion of the hollow hole with an inside diameter of 2 to 3 mm as in the case of the fuel injection nozzle, electrochemical machining has been performed conventionally.
Methods other than electrochemical machining include methods proposed in Patent Documents 1 to 3. Patent Document 1 has disclosed a technique in which a material is formed into a cup shape and the upper end peripheral edge of the cup-shaped material is expanded to the outside, and then the expanded upper end peripheral edge is projected to the inside by ironing from the outside using a die, resulting in the formation of an undercut on the inside of the material.
Patent Document 2 has disclosed a technique in which a rod-shaped material is put into a die whose inside diameter in the upper end portion thereof is larger than the diameter of the rod-shaped material, and the upper end of the rod-shaped material is pressed from the upside by using a punch having a diameter smaller than that of the rod-shaped material, by which the diameter of the upper end portion of the material is increased following the die shape, and an undercut is formed automatically when the small-diameter punch advances into the upper end of the rod-shaped material.
Patent Document 3 has disclosed a technique in which a material having a step portion that comes into contact with the shoulder portion of a die is set in the die having the shoulder portion, and a mandrel is inserted to an intermediate position of a blind hole formed in the material, and then the material is swaged by a punch in this state, by which a material in the upper half portion of die is deformed, and at the same time, an undercut is formed in the lower half portion of die without producing a flow of material to the inside in the radial direction.
Also, in an automotive engine, a slender, cylindrical valve guide is installed on a cylinder head to guide the reciprocating linear motion of valve stems of an intake valve and an exhaust valve. As a material for the valve guide, an iron sintered material or a copper-base alloy is generally used. However, with increasing engine output, there has been proposed the use of a material that is light in weight and has high heat resistance.
Also, a small-diameter guide hole is formed in the valve guide installed on the cylinder head of engine, and the valve stem of the intake valve or the exhaust valve is inserted through this guide hole so as to slide at a high speed and be used at high temperatures. Therefore, the valve guide is required to have high wear resistance, seizure resistance, scuff resistance, and heat conductivity.
To meet the above-described requirements, a sintered material of Fe alloy has conventionally been used as a material for the valve guide. However, this material has a drawback of increased weight.
Thereupon, Patent Document 4 has proposed a method in which a molten aluminum-silicon alloy is quenched, solidified, and deposited while being gas atomized to manufacture an ingot, and the ingot is extrusion molded into a tubular shape and is cut to a predetermined size, by which a valve guide is obtained.
Also, Patent Document 5 has proposed a method in which although not limited to the method of forming the valve guide, a molded product obtained by preforming quenched and solidified aluminum alloy powder at a temperature not lower than ordinary temperature and not higher than 300° C. is forged at a temperature of 450° C. to 540° C. as a method of manufacturing an aluminum alloy having high heat resistance.    Patent Document 1: Japanese Patent Application Publication No. 56-59552    Patent Document 2: Japanese Patent Application Publication No. 3-207545    Patent Document 3: Japanese Patent Application Publication No. 8-90140    Patent Document 4: Japanese Patent Application Publication No. 11-350059    Patent Document 5: Japanese Patent Application Publication No. 6-145921
In the case where a member having an undercut is manufactured by electrochemical machining, a cleaning process is always necessary, and a problem of disposal of waste liquid produced by polishing etc. occurs.
On the other hand, in Patent Documents 1 to 3, the location at which the undercut is provided is restricted. That is to say, in Patent Document 1, the undercut is formed in the whole of material, in Patent Document 2, the location at which the undercut is provided is restricted to the upper end portion of material, and in Patent Document 3, the location is restricted to a far portion of the hole formed in the axial direction.
Also, in all of Patent Documents 1 to 3, since the undercut is formed by bending the material itself, it is difficult to make the shape of undercut fixed, and the product yield is lower.
On the other hand, regarding the valve guide, as described above, the quenched and solidified aluminum alloy powder obtained by the atomization process as in Patent Documents 4 and 5 has high wear resistance, heat resistance, and seizure resistance, so that the use of this powder as a material for the valve guide of engine etc. reduces the weight.
However, the quenched and solidified aluminum alloy powder is not only high in cost but also unsuitable for forming a tubular member having a small-diameter guide hole as in the case of the valve guide because of the difficulty in machining. That is to say, the tubular member is manufactured by hot extrusion, so that the service life of die is short and the energy for heating is required, which poses problems in terms of equipment and cost.